As described in published U.S. Patent Application Number 20060034664, conventional gas extraction wells at landfills often involve deep wells attached to a network of pipes and a gas pump (blower) that applies vacuum to extract the gas from the stored waste. The profile of surface emission flux is recognized to lead to potential for some emissions away from the wells under most circumstances. Note also that there is almost always entrainment of gas, whether LFG or atmospheric air, through the surface area most proximate to deep collection. Both LFG emission far from wells, and air entrainment proximate to subsurface collection, are well recognized as deleterious to collection efficiency. A “tradeoff” exists between extracting or “pulling” at too high a flow rate and entraining excessive atmospheric air, and pulling too little and recovering less LFG. This poses one dilemma of conventional extraction.
A prior art arrangement according to the above published patent application is shown in FIG. 1. Landfill 1 containing waste 2 generates biogas (biogas flows shown by the arrows). Biogas is collected and extracted through well 3. The well 3 includes a gas-collecting well screen 16 and a gas-impermeable conduit 17 linking the well screen to the surface to draw biogas from the wellhead to the surface. Overlaying the majority of the waste 2 is a gas-permeable layer 5. The term “wellhead” refers to a portion of the gas-extraction well where gas can be extracted therefrom. The well often includes a section of pipe having slots or other gas-flow apertures cut in it, referred to as a “well screen”. Often, the well screen is also surrounded with gravel. The gas-permeable layer is typically composed of a conductive porous matrix with gas flow paths. Often it is composed of rigid or semi-rigid particles of a large enough size to leave a significant void volume between particles. For instance, the gas-permeable layer may contain sand, gravel, wood chips, or shredded tires. Above the gas-permeable layer is a gas-containment layer 7. Biogas that rises from the landfill reaches the gas-permeable layer where it is trapped by the overlying gas-containment layer 7. The biogas migrates horizontally in the gas-permeable layer until it comes close to a well. Gas extraction from the well creates a vacuum that draws gas into the well. This vacuum draws biogas from the overlying gas-permeable layer down through the waste mass of the landfill to reach the well. The area immediately beneath the gas-permeable high conductivity layer 5 through which a substantial fraction of the biogas from the gas-permeable layer passes as it travels to the gas-collection wellhead is the entrainment zone 9. On its passage through the waste 2, the gas from the gas-permeable layer mixes with biogas produced in the waste mass that has not gone through the gas-permeable layer. This helps to give a consistent content to the biogas that is withdrawn from the well. If gas is withdrawn directly from the gas-permeable conductive layer 5 (e.g., through conduit 8), the gas composition will vary more dramatically over time, sometimes containing a high air content and sometimes not. It is sometimes desirable to place an even more impermeable layer, such as geomembrane 15, directly over the zone of entrainment of gas from the permeable layer that is created by the deep well. Moreover, sometimes the entire landfill is covered with such a membrane.
FIG. 2 shows another prior art arrangement, this time showing a more shallow wellhead 26 used to withdraw near-surface or sub-surface gas from beneath a membrane M capping a waste W. The wellhead 26 is attached to an above-ground conduit C by way of a vertical pipe. Where the pipe extends through the membrane M, such is prone to gas leakage out and/or air leakage in (depending on the relative pressures in the waste W and the atmosphere). To address this, it has been known in the prior art to install a polymer boot B which typically is bonded (welded or glued) to the membrane M and bonded or clamped to the pipe P. Unfortunately, such boots are rather prone to leakage and the seal provided thereby is less than ideal.
Accordingly, it can be seen that there exists a need for a better way for extracting sub-surface gas from near the surface of landfills. It is to the provision of solutions to this and other problems that the present invention is primarily directed.